Page party systems for use in industrial environments, such as power plants, are generally well known in the art.
Older prior art page party system typically include a page line and a single party line per system. The party line typically comprises a single twisted-pair. When the page line is activated, a caller may page several individuals over one or more loudspeakers located throughout the plant. The call recipients may then pick up handsets at various stations and communicate with the calling party over the single twisted-pair line at the baseband frequency (i.e. voice signals are not modulated). As other parties are paged, they, too, may pick up handsets and communicate over the same twisted-pair line, thereby creating a full-duplex conference call. Full-duplex conferencing is realized by placing the voice signals of all parties on the twisted pair via individual current sources and passively summing these currents with a terminating impedance. Summing is permissible at the baseband frequency because the voice signals are linear. However, only one conference call may be handled at a given time on such a system because only a single party line (i.e. twisted pair) connects all stations.
In certain later page party systems, multiple simultaneous conference calls are possible. In these later systems one page line is provided as in the case of the earlier systems. However, unlike earlier systems, more than one party line is provided. In other words, in these later systems, more than a single twisted pair is used to connect the various stations. In these later systems, multiple conference calls can occur simultaneously, i.e. one conference call per twisted wire pair.
It would be desireable to provide a multiple simultaneous conferencing capability to those older systems that do not already have such a capability.
Additionally, it would be desirable to provide such a capability without the necessity of recabling so as to make additional twisted pair cable runs between stations.
U.S. Pat. No. 4,847,888--Cox (the "Cox patent") is generally directed to a page party system that purportedly allows for multiple simultaneous conference calls over a single twisted pair. In fact, however, while the system of the Cox patent does permit more than one multiple party call to occur simultaneously, it does not permit true conferencing to occur. A true conference call is one in which all parties can simultaneously talk and listen to the entire conversation of all parties to the call. True conferencing is known as "full-duplex" communication with two or more parties. The system of the Cox patent does not permit full-duplex communication.
The system of the Cox patent transmits voice information in FM from station to station on a generated carrier. Thus, in the system of the Cox patent, after a party is paged, the call recipient depresses a button on his handset and a high frequency carrier signal is generated. The call recipient's voice is then frequency modulated on the carrier signal for transmission to the calling party, where it is received and demodulated. In addition to reducing noise, the modulation of voice information at higher frequencies enables the system of the Cox patent to provide more than one multiple party call to occur simultaneously over the same twisted pair. Different multiple party calls may simultaneously take place on the same twisted-pair as long as each multiple party call uses a different carrier frequency to transmit voice information. The carrier frequencies are selected such that the different multiple party calls occupy adjacent, non-overlapping frequency bands on the single twisted-pair. This technique, called frequency division multiplexing (FDM), is well known to those skilled in the art.
True conferencing is achieved in older page party systems by simultaneously placing the baseband voice signals of each speaker on the twisted-pair via current sources and passively summing these currents with a terminating impedance. Summing is possible because the voice signals at the baseband frequency are linear signals. The system of the Cox patent, however, is incapable of true conferencing because frequency modulated carriers, such as those used in the Cox system, are non-linear and cannot be summed. As a result, at a given carrier frequency, only one modulated carrier signal (i.e. one speaker) can be transmitted at a time. In this regard the device of the Cox patent operates somewhat in the manner of a "walki-talki." While all parties to a particular conference call may listen to a speaker, only one person may speak at a time. Therefore, although the Cox patent describes a purported multiple conference call capability, true conferencing is not permitted with the system of that patent.
In order to provide true conferencing in the system of the Cox patent, a conference bridge might conceivably be employed. With a conference bridge, however, each speaker is connected to a central point, or "bridge" via his own twisted-pair line. At the conference bridge, the frequency modulated carrier signals of each speaker are demodulated, thus reproducing the voice information of each speaker at the baseband frequency. These baseband voice signals are then summed and remodulated on a single carrier for transmission to each station over an existing single twisted-pair. While such an approach would allow for full-duplex communication, two significant drawbacks would be introduced by such a system. First, each speaker must be connected to a conference bridge, and therefore, if the conference bridge fails, the whole system fails. Second, each speaker in the conference occupies a unique channel. Consequently, the number of channels available is directly proportional to the number of people in the conference.
It would be desireable, therefore, to provide a page party system that provides for simultaneous, multiple, full-duplex conference calls over an existing single twisted-pair, without the necessity of a conference bridge. The page party system of the present invention achieves the foregoing object.